The present invention relates to an apparatus for storing contamination-sensitive flat articles, in particular for storing articles used in the production of semiconductor products and more particularly for storing semiconductor wafers.
The production of large-scale-integrated electronic circuits and other sensitive semiconductor components is nowadays carried out in factories in which so-called semiconductor wafers pass through a plurality of processing steps. Semiconductor wafers are circular disks composed of a semiconductor material, such as silicon, on which microstructures are formed by various exposure, etching and doping processes. The wafer is typically coated with an oxide layer and with a light-sensitive varnish arranged above the oxide layer. The varnish is then exposed through a photomask (the “reticle”). The photomask images the desired microstructure on the wafer. After a development step, the photoresist is removed at the unexposed points, exposing the oxide layer. In a subsequent etching process, the exposed oxide layer (and only this layer) is removed. The semiconductor material, which is now exposed in places, can then have its material characteristics changed by doping processes (deliberate introduction of different material), thus resulting in the desired microstructures. These process steps are typically carried out more than once in different versions until a large-scale-integrated electronic circuit or, for example, a liquid crystal component, has been manufactured.
The majority of these process steps take place in clean-room conditions, i.e. in environmental conditions which are kept as free as possible from impurities, foreign substances and the like. This is because any contamination of the wafer by other materials can change its material characteristics undesirably, and this can lead to an entire production batch being unusable.
For an efficient production, it is necessary to temporarily store the semiconductor wafers before and after the individual process steps are carried out. Furthermore, semiconductor wafers also have to be stored for other reasons, for example as a stock and as initial material for a new production batch, as “filling material” in order to fill only partially filled production installations (in order, for example to ensure a defined temperature distribution in an oven), or as test wafers for testing a production process. It is important to ensure that the wafers are kept free from contamination during storage times.
In modern factories, wafers are often transported between individual production installations in special transport boxes, which are know as FOUPs (Front Open Unified Pots). Wafers are often also stored in these FOUPs, which have a special cover for closure. The internal and external dimensions of the FOUPs are exactly specified, since automated handling systems are typically used for loading, unloading and transport of the FOUPs. When wafers are stored in FOUPs, the internal and external dimensions of the FOUPs determine the space required and the storage capacity. A storage system for wafers using FOUPs is disclosed, for example, by EP 1 197 990 A2.
DE 103 29 868 A1 discloses a storage system for wafers, wherein the wafers are stored in a horizontal orientation on a carousel, which is kept in clean-room conditions. Horizontal storage of the wafers corresponds to the practice in FOUP-based storage systems, because the automated handling systems for FOUPs are typically configured to insert and remove the wafers in horizontal orientation.
US 2002/0094257 A1 discloses a storage system for reticles, wherein the individual reticles are held in a vertical orientation on carousels arranged one above the other. This document states in an extremely general form that such a system may also be used for storing semiconductor wafers, but without discussing any details or observing differences between wafers and reticles. For example, a reticle has to be protected “only” against dust and other physical particles which can lead to corruption of the microstructure that is imaged in the exposure step. In contrast, semiconductor wafers must also be protected against gaseous substances, which can introduce undesirable doping atoms into the semiconductor material. Furthermore, storage systems for reticles require different handling and storage systems in comparison to wafers, because of the different geometric dimensions.
The storage of wafers and other contamination-sensitive articles outside of FOUPs and other transport containers involves a risk of a single contaminated article contaminating a large number of other articles stored in the same area. On the other hand, the storage of wafers and other contamination-sensitive articles in FOUPs and other transport containers requires a large amount of storage space. This is particularly problematic, because the wafer size in modern semiconductor factories is increasing, because increasingly larger semiconductor wafers are used. By way of example, modern semiconductor factories use wafers with a diameter of 300 mm and require correspondingly large FOUPs, while older production installations generally use 200 mm wafers.